The present invention relates to a method for conducting the solutions of liquid-liquid extraction, which are mixed into dispersion in a mixing tank, from the mixing tank further to a settler. The dispersion created in the mixing tank is fed, by means of the method and apparatus of the present invention, into the settler through distribution channels arranged along the whole width of the settler to extend over the whole width of the settler before said dispersion is made to be discharged into the settler proper.
As regards the liquid-liquid extraction solutions, the organic solution is generally composed of a kerosene-type hydrocarbon and an extraction agent proper dissolved therein, and the second solution is an aqueous solution. The solutions are mixed into dispersion in a mixing tank, which can constitute one or several mixers. From the last mixer, the dispersion is conducted to a settler where it is divided into two separate phases, and the phases are classified as two superimposed layers on the basis of gravity.
From the Fl patent 96968, there is known a method according to which from the last mixer of the mixing tank there is access to an uptake shaft which opens in the front wall of the settler, and through which the dispersion is set to enter the settler as a horizontal flow.
As regards very wide settler arrangements, a dispersion fed in through the middle of the settler is not evenly distributed over the whole cross section of the settler, but side currents along the feed edge of the settler become so strong that the situation is difficult to control solely by means of picket fences which are normally used for guiding the flow. Moreover, it should be taken into account that the flow quantities may vary, which also causes unstability.
As for the settler feeding arrangements, it has been troublesome to turn over to larger solution flows, because along with larger total flows, the bottom of the mixing tanks must be built further and further down, and consequently further from the settler bottom level. This is due to the fact that mixer dimensions are designed according to a given solution delay time, and settlers are respectively designed on the basis of a given settling capacity determined according to area proportions. With large total flows, this means that the mixing tank bottoms are 2-5 m lower than the settler bottom.
A liquid-liquid separation carried out under the influence of gravity requires a settler with an area that grows directly in proportion to the solution quantities to be separated. For instance in copper extraction, in order to separate said solution quantities form each other, there is needed an area of 0.2 m2 per each solution cube separated in an hour (m3/h). However, this requires that there are no disturbing turbulent circulations in the settler, because said circulations easily lower the separating capacity of the settler. From the point of view of separating the solutions, it is advantageous that there is achieved an even plug-like flow in the settler over the whole cross sectional area flow. It has also been observed that the linear rate of the solution must not exceed a certain critical limit, if the amount of the residual drops of one solution contained in the other should be kept low.
In copper extraction, the linear speed limit for the organic solution is considered to be 6 cm/s, and in large plants, this is easily exceeded. The layer thickness of the organic solution is generally not desired to be raised above 30 cm, because when the layer thickness grows, also the quantity of the expensive extraction agent in the solution grows, and this increases the costs. Thus the widening of the settler is an important way to prevent the linear speed from growing too high. The need for widening the settler also is apparent from the appended table that shows how the settler width develops when the employed layer thickness for the organic solution is 30 cm, and the linear speed is kept at the value of 5.5 cm/s.
The present invention relates to a method and apparatus for conducting the dispersion evenly to a wide settler, particularly a settler that is more than 25 m wide, although it is obvious that the arrangement can also be used in narrower settlers, where a fluent and correct operation of the settler should be ensured. In the method according to the invention, the dispersion coming from the mixing part is conducted from downwards to the liquid part of the settler, advantageously to the middle region of the feed end, wherefrom it is made to be distributed to inside the settler, in a channel-like closed space along the whole width of the settler, placed in the immediate vicinity of the settler front edge, and to flow therefrom, mainly above the channel-like part, towards the rear end of the settler. The method according to the invention is realized by means of an apparatus where an uptake shaft leading from the dispersion mixer part to the settler is opened towards the settler feed end, from where there are further directed distribution channels towards the settler side edges. The distribution channels are located in the bottom part of the settler. The essential novel features of the invention are apparent from the appended claims.